As is well known, bobbin cases are commercially available, on the bottom of which a shaped resilient or spring-like thin plate is fastened, this thin plate forming a braking spring which is effective to prevent an undue feed or advancement of the bobbin under the thread traction, which would otherwise cause a breakage in the thread. Depending on the thread being used, the pressure of such a braking spring has to be suitably regulated by the action of an adjusting screw running in a threaded hole in the bobbin case with one of the ends of this screw pressing perpendicularly against the braking spring. However, it is known that the bobbin cases are alternately subjected to vibrations at each of the thread passages. Due to these stresses, the pressure spring adjusting screw in the threaded hole of the bobbin case is urged to a great extent, so that the position of said screw is varied with the different stitching speeds. Accordingly, the pressure on the bobbin braking spring, which consists of a resilient thin plate secured at one of its ends to the bobbin case, will vary as a function of the above-mentioned factors and, therefore, new adjustments are repeatedly required. Additionally, such drawbacks in the bobbin as those causing the lower thread to "jump" beyond the bobbin flange are not avoided, and as a result, a breakage in the thread takes place.
It is an object of the present invention to provide a bobbin case of the above-mentioned design, wherein the adjusting screw for the bobbin braking spring is prevented from loosening or generally changing its position during the operation of the sewing machine.
This object is accomplished by using means for exerting a pressure on the adjusting screw of the braking spring which is perpendicular to the axis of the screw, so that an even, constant loading can be imparted. Thus, following regulation of the pressure on the bobbin braking spring by means of said adjusting screw, the latter invariably maintains its position, so that the braking spring presses evenly on the bobbin.
Accordingly to a particular embodiment of the invention, it is contemplated that a thrust means be used which substantially comprises a plunger urged by a spring against said adjusting screw. Preferably, such a spring may be the spring normally used for the return of the bobbin case latch; therefore, the said thrust means performs a dual function, i.e., it acts as an abutting element for the latch spring and operates as a means for exerting a clamping pressure on the adjusting screw.
The thrust means may be made from any suitable material whose hardness is not unduly high, such as (plastic) synthetic material, or metal material having substantially similar characteristics.
The hole or bore for the spring and the associated thrust means is generally formed at an offset position relative to the axis of the adjusting screw, thus forming a side shoulder limiting the forward stroke of the thrust means; accordingly, the threaded hole is not obstructed and the adjusting screw can be easily and freely removed and threaded inwardly again.
According to another embodiment of the invention, the thrust means may be formed as a leaf spring which is inserted into a side housing opening to the threaded hole for the adjusting screw, and the leaf spring is suitably shaped to press laterally and perpendicular to the axis of the screw.